Method of producing metal alloys having a high nitrogen content



United States Patent 3,357,827 METHOD OF PRODUCING METAL ALLOYS HAVING A HIGH NITROGEN CONTENT Gerhard Naeser, Werner Schulz, Otto Wessel, and Norbert Dantzenberg, Duisburg, Germany, assignors to Mannesnlann Aktiengesellschai't, Mannesmannufer, Dusseldorf, Germany, a corporation of Germany No Drawing. Filed May 31, 1966, Ser. No. 558,540 Claims priority, application Germany, June 2, 1965, M 65,428 7 Claims. (Cl. 75-224) ABSTRACT OF THE DISCLOSURE This process aims at producing chromium containing steel alloys with high nitrogen content. It achieves its aim by melting the chromium containing steel alloy, forming from the melt a powder by atomization and annealing the powder at atmospheric pressure in a nitrogen atmosphere at temperatures ranging between about 800 C. and 1100 C. Prior to the annealing step in the nitrogen atmosphere it may be advantageous to compress the powder to a porous body. The compression step may be carried out by simple pressure or by rolling. It may also be advantageous to introduce another annealing step prior to the nitrogen atmosphere annealing by exposing the powder, either in its powder form or after compression as a porous body, to reduced pressure (vacuum) at temperatures exceeding the maximum temperature used in the nitrogen atmosphere annealing, which is 1100 C. This prior annealing step reduces the oxygen and carbon content of the alloy prior to the contact with the nitrogen atmosphere. Suitable illustration for the temperature used in the prior annealing step is, for example, 1150 to 1200 C.

By this process nitrogen contents of up to 2%, or even more, can be produced, whereas prior art processes ranged from about 0.2% to about 0.8% nitrogen. Nickel may be present in the chromium containing alloys and the high nitrogen content permits the reduction of the nickel content without adverse effects.

The invention relates to the production of metal alloys having a high nitrogen content.

While the presence of nitrogen in pure carbon steel is looked upon as disadvantageous, nitrogen has in a variety of metals achieved importance as an alloying element. A nitriding with ammonia at relatively low temperatures leads to hard wear-resistant surfaces that furthermore improve the corrosion resistance, particularly at porous sintered parts. It is furthermore possible to substitute in chromium nickel steels nitrogen at least in part for the expensive nickel, and to achieve thereby an improvement in heat resistance.

In the past it has been proposed to produce metal alloys having a high nitrogen content by methods as follows: Adding to the metal bath of salts, for instance calcium cyanamide, which give off nitrogen at high temperatures; use of nitrided alloying elements, such as ferrochromium; or melting of the metal under increased nitrogen pressure. The first and second methods lead to melting with nitrogen contents that correspond to air saturation, for instance leading at the melting of an 18-8 chromium nickel steel to a nitrogen content of about .2%. By melting and pouring under a nitrogen pressure of twenty atmospheres above the normal atmosphere, on the other hand, in accordance with the third method, nitrogen contents of up to .8% nitrogen have been achieved, for instance at 18-8 chromium nickel steels, owing to the dependency of the pressure from the saturation concentration.

The nitrogen concentrations achieved with the first two mentioned methods are for most purposes too low. The melting under high nitrogen pressure, on the other hand, under the third method, is very expensive, particularly when even higher nitrogen pressures above the aforesaid twenty atmospheres overpressure need to be used to achieve a nitrogen content of about the aforesaid .8%.

It is accordingly among the principal objects of the invention to provide a method of producing metal alloys having a high nitrogen content, that avoids the drawbacks of the prior art.

In accordance with the instant invention, metal alloys. particularly iron and steel alloys may be produced having a high nitrogen content, preferably above .5 The instant invention provides for pulverizing a metal to obtain solid metal powder, for instance by pulverizing a molten metal alloy with pressurized water; thereafter, either the powder or a body formed by pressing or rolling from the powder, for instance a block or a sheet of metal, is subjected to a decarburizing and/ or reducing annealing at a rarefied atmospheric pressure at a temperature of above 1100 C.; immediately after the cooling to a temperature of between about 1100 C. and about 800 (1., preferably at normal atmospheric pressure, the powder or body, respectively, is annealed in a nitrogen containing atmosphere.

It is a further object of the invention to provide metal alloys with nitrogen contents of a large amount heretofore unobtainable. It is, for instance, possible in accordance with the invention to achieve 18-8 chromium nickel steel alloys with a nitrogen content of up to 2%.

It is another object of the invention to provide for compression and further working of either the powder or the body formed therefrom after the nitriding; this compression, respectively further working may be by means of well-known pressing or rolling, preferably at a high temperature from about 1000" C. to about 1200 C.

The invention accordingly comprises the several steps and the relation of one or more of such steps with respect to each of the others, all as exemplified in the following detailed disclosure, and the scope of the application of which will be indicated in the claims.

As previously mentioned, either the powder or the body which is formed from the carbon containing powder is nitrided for achieving a desired nitrogen content only after a preceding decarburizing annealing in a vacuum. This may be done in such a manner that after the vacuum annealing, that is carried out preferably at a temperature range of between about 1150 C. and 1200 C., nitrogen gas is led into the annealing furnace, which is still at a high temperature though its temperature has receded to a range of from about 1100" C. to about 800 C.

The decarbuizing first annealing step serves the following purposes: The surfaces of the grains of the powder can absorb the nitrogen during the subsequent annealing in the nitrogen atmosphere faster, so that there will be achieved already within a few minutes the desired nitrogen content in uniform distribution.

EXAMPLE I For producing a chromium containing steel alloy with a high nitrogen content, the porous body formed of the steel powder will be annealed, in the aforesaid second nitrogen annealing step, in accordance with the invention, at a temperature of between 800 C. and 1100 C., preferably 950 C., with the aid of a gaseous nitrogen at a pressure of about one atmosphere.

EXAMPLE II The table below shows the results of several nitridings of various chromium containing steels, in accordance with the invention. The table contains the analysis of the steel alloys and their nitrogen contents after an annealing of three hours at 950 C. with nitrogen gas:

Per- Per- Per- Per- Per- Percent cent cent cent cent cent Si Mn Cr Ni N Steel:

A 12 1. 52 38 18.0 9.9 2. 3 B .11 1. 55 .26 12.1 1.6 C l5 1. 68 .61 17.3 2.5

EXAMPLE III For the production of tubes of 18-8 chromium nickel steel with a high nitrogen content, there was first produced a steel powder by atomizing the steel melt with pressure water, with a stoichiometrical relation of carbon to oxygen, which was thereafter pressed into blocks each weighing 20 kilograms at a pressure of 2 tons per square centimeter. The blocks were subsequently annealed in vacuum at 1150 C. for eight hours, for removing the carbon and oxygen. Thereafter, the furnace was cooled to 950 C. and the nitrogen was admitted. After an annealing time of twenty minutes, these blocks showed throughout their entire cross section uniformly distributed a nitrogen content of .8%. Subsequently, the blocks were heated under nitrogen to 1150 C. and shaped into tubes in an extrusion press.

While normal 18-8 steel at 600 C. has a heat yield point of from 8 to 12 kp. per square millimeter, it had for the nitrided steel at the same temperature such a heat yield point of about 38 kp. per square millimeter. The toughness characteristics of the nitrogen containing steel were only a little lower as compared to the nitrogen free steel.

We Wish it to be understood that we do not desire to be limited to the exact details of construction shown and described, for obvious modifications will occur to a person skilled in the art.

Having thus described the invention, what we claim as new and desire to be secured by Letters Patent, is as follows:

1. In a process for producing chromium containing steel alloys with high nitrogen content, the steps comprising (a) melting the metal,

(b) producing a metal powder by atomizing the molten metal and (c) annealing the so produced metal powder at atmospheric pressure in the presence of gaseous nitrogen at temperatures ranging from about 800 C. to about 1100" C.

2. The process of claim 1, wherein the annealing temperature is about 950 C.

3. The process of claim 1, wherein the atomizing in step (b) is carried out by the aid of pressurized water.

4. The process of claim 1, wherein the metal powder formed in step (b) is compressed by mechanical means into a porous body prior to the annealing step of (c).

5. The process of claim 1, wherein the metal powder formed in step (b) and prior to the annealing step of (c) is exposed to another annealing step at pressures reduced to less than an atmosphere and at temperatures higher than the top temperature limit in the subsequent nitrogenous annealing step of (c), thereby reducing the carbon and oxygen content of the chromium containing steel alloy.

6. The process of claim 1, wherein the metal powder formed in step (b) is compressed by mechanical means into a porous body and prior to the annealing step of (c) is also exposed to another annealing step under vacuum and at temperatures exceeding that of the annealing step of (c) in order to reduce the carbon and oxygen content of the alloy.

7. The process of claim 4-, wherein the compressing step is carried out by rolling.

References Cited UNITED STATES PATENTS 2,933,386 4/1960 Pessel 75-224 2,989,429 6/ 196 1 Tangzyn 148-126 X 2,994,600 8/1961 Hansen 75211 X 3,161,949 12/ 1964 Dickinson 75-224 X FOREIGN PATENTS 899,915 6 1962 Great Britain.

. CARL D. QUARFORTH, Primary Examiner. 

1. IN A PROCESS FOR PRODUCING CHROMIUM CONTAINING STEEL ALLOYS WITH HIGH NITROGEN CONTGENT, THE STEPS COMPRISING (A) MELTING THE METAL, (B) PRODUCING A METAL POWDER BY ATOMIZING THE MOLTEN METAL AND (C) ANNEALING THE SO PRODUCED METAL POWDER AT ATMOSPHERIC PRESSURE IN THE PRESENCE OF GASEOUS NITROGEN AT TEMPERATURES RANGING FROM ABOUT 800* C. TO ABOUT 1100*C. 